At the present time, two-piece can-type containers are widely used for beverages such as beer and soft drinks. A two-piece can container comprises a drawn and ironed one-piece can body member and a one-piece end or lid member having an easy-open device mounted thereon. The can body member has a cylindrical side wall portion, a bottom wall portion and an open end necked and flanged rim wall portion for attachment of the end member after filling of the can body member. The bottom wall portion has a convex annular lowermost reduced diameter support rib portion circumjacent a dome-shape axially inwardly extending concave central panel portion to provide sufficient strength to enable use of less costly thinner gauge sheet material for the can body member. Relatively small reductions in the size and gauge of the sheet material result in very substantial cost of material savings when billions of cans are involved.
A typical aluminum can body member of current design is made from 0.012 inch thickness aluminum alloy sheet material and has a reduced thin side wall portion of approximately 0.004 inch thickness. The bottom wall portion is approximately 0.012 inch thick and is connected to the thin side wall portion by a tapered transition wall portion. The upper rim wall portion is approximately 0.006 to 0.007 inch thick to enable seaming (attaching) of the lid and is connected to the thin side wall portion by a tapered transition wall portion.
Some of the limitations on size and gauge of the sheet material are that the can body member must provide (1) a predetermined volume (e.g. 12 ounces); (2) sufficient strength to enable high speed manufacturing operations such as trimming and necking and flanging of the rim portion, decoration, internal coating, filling and attachment of the end member by seaming; (3) sufficient side wall strength to prevent damage during handling, manufacturing, filling, transportation, storage and use; and (4) structural integrity such as to prevent leakage and deformation when filled with carbonized beverages.
While the use of a domed bottom wall structure enables reduction of the can bottom wall thickness, such structure also reduces volume so that can height may have to be increased. Reductions in depth of the domed bottom wall structure are advantageous so as to increase volume and reduce can height. However, the domed bottom wall structure must have sufficient strength and structural integrity to prevent bulging and reversal under pressure which may exceed 100 psi with carbonated beverages subject to relatively high temperatures and agitation.
A can body member must be designed to withstand a certain maximum inside pressure and to provide a certain minimum column strength to prevent buckling under axial loads during shipping and handling. If the side wall portion is too thin, the cans at the bottom of a vertical stack of cans, such as on a pallet, may buckle. Typically, the selected wall thickness is more than is required for internal pressure requirements in order to prevent buckling.
Can failure due to internal pressure occurs most often at the bottom wall portion when the "dome" reverses. If the dome wall portion is made of too thin material, the dome portion will deform downwardly under internal pressure and the can will rock if placed on a flat surface, such as a table. Normal internal pressure is caused by carbonation in the beverage and/or heat of pasteurization. In use, the internal pressure may be greatly increased when the beverage is subject to high atmospheric temperature and/or agitation. Most can designs also make the dome side wall almost vertical to enhance dome strength and the circumjacent convex support rib has a diameter smaller than the can side wall diameter in order to enable stacking of one can on top of another can.